the TP, all the three phases coexist. Moving along the gas-liquid equilibrium curve, we reach a point where a pure gaseous compound cannot be liquefied regardless of the pressure applied. This is the critical point for the compound. The temperature at the critical point is the critical temperature, and the vapor pressure of the gas at the critical temperature is the critical pressure. A supercritical fluid (SCF) is any compound at a temperature and pressure above the critical values. In Figure 11.12, C is the critical point at the end of the gas-liquid equilibrium curve and the shaded area indicates the supercritical fluid region.
In the supercritical environment only one phase exists. The fluid, as it is termed, is neither a gas nor a liquid and is best described as intermediate to the two extremes. This phase retains solvent power approximating liquids, whereas the penetration power into the solid matrix is contributed by the transport properties common to gases. Therefore, the rates of extraction and phase separation are significantly faster than for conventional extraction processes. Furthermore, the pressure and temperature of the fluid in the supercritical region can be altered to effect selectivity in the extraction.
Carbon dioxide is the most commonly used supercritical fluid for spice extraction; primarily due to its low critical parameters (31.1°C, 73.8 bar), low cost, and nontoxicity.
Spice extraction using supercritical carbon dioxide (SCO2) has the following advantages over conventional solvent extraction:
• Supercritical operation provides greater flexibility since the solvation power and selectivity for the solutes can be easily manipulated by altering the temperature-pressure conditions. This facilitates the recovery of products with predesigned physicochemical properties.
• Conventional extraction media are organic solvents with varying levels of toxicity. Even with the most efficient solvent-removal techniques, residual solvents in parts per million (ppm) levels are likely to be present in the end product. SCO2 extraction eliminates such solvent residue problems.
• Carbon dioxide is nonflammable, noncorrosive and nontoxic. Hence, storage and handling do not pose any health hazard.
• It is a part of the environment and hence does not precipitate any environmental issues.
• CO2 is relatively less expensive. It can be recycled and reused in the system.
• The operation can be carried out at lower temperature, thus preserving the heat-sensitive flavor components.
• CO2 is chemically inert and does not react with the components of the solute. Moreover, CO2 can act as a blanket and prevent oxidative degradation of these components.
However, the extractor and accessories for supercritical fluid extraction have to be designed to operate under high pressure. A schematic layout of supercritical extraction assembly is displayed in Figure 11.13. The pump draws liquid CO2 from the collection/storage vessel, compresses it to the required pressure, and transfers it to the extraction vessel through a heat exchanger where it is heated to the extraction temperature. The CO2 containing the dissolved product is directed to the separator. The pressure and/or temperature of the CO2 is reduced in the separator, causing the product to become separated. Several separators maintained at progressively decreasing pressure/temperature might be used so that fractions of the extract with different qualities can be collected
separately. The gaseous CO2 from the separator is liquefied in a refrigerated heat exchanger and collected in the collection/storage vessel for recycling.
In fresh ginger, the pungency is almost entirely contributed by gingerols (Nambudiri et al., 1975; McHale, et al., 1989). Gingerols are homologues of 1-(3-methoxy-4-hydrox-yphenyl)-3-keto-5-hydroxyhexane (Zhang et al., 1994). The major homologues identified are 6-, 8-, and 10- gingerols (Chen et al., 1986c). The prefixes indicate the length of the alkyl chain of the aldehydes—hexanal, octanal, and decanal—that would be obtained by alkaline fission of the gingerols (Connell and Sutherland, 1969). Gingerols are thermally labile and can undergo changes during processing and storage. Two degeneration pathways have been established (Connell, 1969; Smith, 1982; McHale et al., 1989; Zhang et al., 1994; Lawrence, 1984):
• Dehydration to shogaols, which is a mixture of the three corresponding homologues.
• Retro-aldol condensation to zingerone, 4-(3-methoxy-4 hydroxyphenyl)-2-butanone), another pungent component, and aliphatic aldehydes that can cause off-flavors.
The oleoresin of ginger contains gingerols as well as the degeneration products. Gin-gerols are the main pungent principles in the oleoresin, amounting up to 20 percent (Natarajan and Lewis, 1980), depending on the variety. The percentage of gingerols also varies with the level of volatile oil in the oleoresin. In stored samples of the oleoresin the proportions of shogaols and zingerone were found to be higher than in newly processed ones, with a corresponding decrease in gingerol content (McHalle et al. 1989). The highly pungent component in gingerols is the 6-gingerol and poorly pungent are 8- and 10-gingerols. Pungency level of shogaols also follow the same pattern (Raghuveer and Govindarajan, 1978). The distribution of the three gingerols is reported to differ widely with the cultivars (Verghese, 1997). Total gingerols in the oleoresin of freshly processed Cochin ginger with 25 to 30 percent volatile oil is usually 14 to 16 percent and the 6, 8- and 10-gingerols are present in the ratio of approximately 4:1:1.2. The transformation of gingerol to shogaol and zingerone is shown in Figure 11.14 (Raghuveer and Govin-darajan, 1978; Chen et al., 1986b). Conversion of gingerols into either of these two compounds indicates loss of quality. Conversion to zingerone seems to be a relatively slow process compared to the shogaol route (Govindarajan and Govindarajan, 1979).
The extent of transformation of gingerols to shogaols depends on the processing conditions. Chen et al. (1986a) analyzed the liquid carbon dioxide extract of freeze-dried ginger by combined thin-layer chromatography-high-performance liquid chromatography-mass spectrometry (TLC-HPLC-MS) and found 6-, 8-, and 10-gingerols to contribute almost the entire pungency. 6-Shogaol was detected only in traces. Other shogaols as well as zingerone were absent. Heat and acid are reported to accelerate the dehydration of gingerols to shogaols (Connell, 1969).
The level of pungency in the spice and its oleoresin are important quality determinants. Traditionally, organoleptic methods have been used for assessing the pungency (Wood, 1987). These methods are, however, highly subjective and yield results that vary considerably. Thin-layer and column chromatography have been suggested for the quanti-
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